Power machineries, such as hydraulic excavators, cranes, wheel loaders, and bulldozers, (in the present specification and claims, these power machineries (heavy machineries) are collectively called “operating machines”) have been used for civil engineering works, construction works, and the like. Taking a hydraulic excavator 100 shown in FIG. 7 as an example, the hydraulic excavator 100 is configured such that: a revolving super structure (structure) 102 is provided on the upper portion of a base carrier 101; and the revolving super structure 102 includes an engine, a driver's seat, an arm 104 having a tip end on which a bucket 103 is provided, a boom 105 coupled to the arm 104, and the like. The boom 105 is configured to be moved up by a boom cylinder 106. Therefore, the revolving super structure 102 is a large heavy structure. By manipulating a remote controller at the driver's seat during operations, the revolving super structure 102 is caused to swing on the upper portion of the base carrier 101. In addition, by driving the boom 105 and the like in a vertical direction, various operations are performed by the bucket 103 provided at the tip end of the awl 104.
Such an operating machine includes a plurality of hydraulic pumps for driving the revolving super structure 102, the boom 105, and the like. The operating machine obtains a high driving force by an operating oil supplied from each hydraulic pump or by causing the operating oils supplied from a plurality of hydraulic pumps to join one another depending on conditions. In recent years, proposed is an operating machine in which the revolving super structure 102 is caused to swing by a driving device including a hydraulic motor and an electric motor.
One example of an operating machine including this type of driving device is an operating machine including a driving device which includes a hydraulic unit having a hydraulic motor as a driving source and an electric unit having an electric motor as a driving source, controls the electric motor by a controller and an inverter at the time of swing, and assists the hydraulic unit by the torque of the electric motor (see PTL 1, for example). According to this operating machine, when the driving device performs steady swing or decelerates, the electric motor is caused to perform a regenerative action, and regenerative electric power is stored in a capacitor. A control unit of the driving device calculates required torque at the time of the swing. When the required torque exceeds a set value, the electric motor outputs necessary torque. To be specific, maximum torque necessary as a whole is secured by causing the electric unit to assist the hydraulic unit. In this case, necessary torque is generated by adjusting the assist amount of the electric unit. The control unit is configured to control the output torque of the electric motor so as to shorten a relief time of a relief valve provided at a hydraulic motor circuit.
Further, another conventional art is a construction machinery including a hybrid driving device having a driving force synthesis mechanism configured to synthesize the driving force of a hydraulic actuator and the driving force of a motor generator. In the construction machinery, a communication valve (bypass valve) is provided for effectively utilizing energy generated at the time of braking, and inertial energy of the revolving super structure is efficiently regenerated as electric energy by the motor generator (see PTL 2, for example). In this conventional art, the relief valve incorporated in the hydraulic motor is used as a setting unit configured to set a ratio between the driving force of the hydraulic actuator and the driving force of the motor generator configured to cooperate with the hydraulic actuator.
Still another conventional art is that: a pressure difference between both ports of the hydraulic actuator is detected; and a torque command is output to the motor generator, provided close to the hydraulic actuator, in accordance with the pressure difference (see PTL 3, for example). In this conventional art, the revolving super structure is configured to be driven by the sum of driving and braking torques of the hydraulic motor and the motor generator. The relief valve configured to control highest driving pressure of the hydraulic motor at the time of driving and stopping is provided such that the ratio of the output torque of the hydraulic motor at the time of the driving becomes larger than that at the time of the braking. The working pressure of the relief valve at the time of the start-up and acceleration is set to be higher than that at the time of the deceleration and stopping.